Phenol and methyl ethyl ketone are important products in the chemical industry. For example, phenol is useful in the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, alkyl phenols, and plasticizers, whereas methyl ethyl ketone can be used as a lacquer, a solvent and for dewaxing of lubricating oils.
The most common route for the production of methyl ethyl ketone is by dehydrogenation of sec-butyl alcohol (SBA), with the alcohol being produced by the acid-catalyzed hydration of butenes. For example, commercial scale SBA manufacture by reaction of butylene with sulfuric acid has been accomplished for many years via gas/liquid extraction.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene relative to that for butenes is likely to increase, due to a developing shortage of propylene. Thus, a process that uses butenes instead of propylene as feed and coproduces methyl ethyl ketone rather than acetone may be an attractive alternative route to the production of phenol.
It is known that phenol and methyl ethyl ketone can be co-produced by a variation of the Hock process in which sec-butylbenzene is oxidized to obtain sec-butylbenzene hydroperoxide and the peroxide decomposed to the desired phenol and methyl ethyl ketone. An overview of such a process is described in pages 113-124 and 261-263 of Process Economics Report No. 22B entitled “Phenol”, published by the Stanford Research Institute in December 1977.
Sec-butylbenzene can be produced by alkylating benzene with n-butenes over an acid catalyst. The chemistry is very similar to ethylbenzene and cumene production. However, as the carbon number of the alkylating agent increases, the number of product isomers also increases. For example, ethylbenzene has one isomer, propylbenzene has two isomers (cumene and n-propylbenzene), but butylbenzene has four isomers (n-, iso-, sec-, and t-butylbenzene). These by-products, especially iso-butylbenzene, have boiling points very close to sec-butylbenzene and hence are difficult to separate from sec-butylbenzene by distillation (see table below).
ButylbenzeneBoiling Point, ° C.t-Butylbenzene169i-Butylbenzene171s-Butylbenzene173n-Butylbenzene183
Moreover, isobutylbenzene and tert-butylbenzene are known to be inhibitors to the oxidation of sec-butylbenzene to the corresponding hydroperoxide, a necessary next step for the production of methyl ethyl ketone and phenol. It is therefore important to ensure that the benzene alkylation process is highly selective to sec-butylbenzene rather than the other isomers.
It is also desirable to minimize the production of other by-products such as butene oligomers, dibutylbenzenes and tributylbenzenes. For example, although the polybutylbenzenes can be converted back to the desired sec-butylbenzene by transalkylation, this represents an additional processing cost. The production of butene oligomers raises even more difficult problems. Firstly, they represent an unproductive loss of the butene raw material, and secondly, the C12 oligomers are extremely hard to remove from sec-butylbenzene by distillation since their volatility is very close to that of sec-butylbenzene. In addition, any of these oligomers that remain in the sec-butylbenzene alkylate are poisons in the subsequent oxidation step. Thus, any process that produces sec-butylbenzene with reduced amounts of polybutylbenzenes and C12 oligomers content is highly advantageous.
In our International Patent Publication No. WO 06/15826 we have described an integrated process for producing phenol and methyl ethyl ketone, in which a feed comprising benzene and a C4 alkylating agent is contacted under liquid phase alkylation conditions with a catalyst comprising zeolite beta or an MCM-22 family molecular sieve to produce an alkylation effluent comprising sec-butylbenzene. The sec-butylbenzene is then oxidized to produce a hydroperoxide and the hydroperoxide is cleaved to produce the desired phenol and methyl ethyl ketone. At least one of the alkylating step, the oxidation step and the cleavage step can be effected by catalytic distillation.
U.S. Patent Application Publication No. 2006/0178544 discloses a method for increasing selectivity of alkylation to monoalkylation comprising: providing a feedstream consisting essentially of alkylating agent and a stoichiometric excess of benzene, the alkylating agent consisting essentially of a molar blend of propylene and one or more linear butene(s); and, contacting the feedstream with a catalytically effective amount of zeolite beta under alkylation reaction conditions which increase selectivity of the alkylation to monoalkylation compared to predicted selectivity to monoalkylation based on the concentration of the alkylating agent and on the molar blend of propylene and one or more linear butene(s). The process may be performed in a fixed bed reactor operating in an upflow or downflow mode or a moving bed reactor operating with concurrent or countercurrent catalyst and hydrocarbon flows. The process may also be performed in a catalytic distillation mode.
U.S. Patent Application Publication No. 2006/0211901 discloses a process for producing cumene and secondary butyl benzene simultaneously in a distillation column reactor by feeding propylene, butylene and benzene to the reactor. Unreacted benzene is removed as overheads and cumene and secondary butyl benzene are removed as products. The catalysts used are acid cation exchange resins or zeolites, particularly beta zeolite.
According to the present invention, it has now been found that when catalytic distillation is applied to the alkylation of benzene with butene, although the competing butene oligomerization reaction also occurs, with proper control of the catalytic distillation process and, in particular of the dimensions of the reaction zone, it is possible to ensure that the sec-butylbenzene and C8 oligomers produced drop quickly from the reaction zone, since they are heavier and less volatile than both benzene and butenes in the reaction zone. Thus the concentration of sec-butylbenzene and C8 oligomers in the reaction zone can be retained at a low level, thereby minimizing their availability for reaction with additional butenes to product polybutylbenzenes and C12=oligomers. As a result the catalytic distillation process can be operated so as to produce a sec-butylbenzene alkylate product that contains very low levels of polybutylbenzenes and C12=oligomers.